The prior art already includes a device for the capacitive measurement of a distance between a sensor head and a workpiece in which a regulated, phase-dependent current source is used. The phase-dependent current source is fed an alternating supply voltage upon which there is superimposed an alternating measuring voltage that is tapped at the sensor head, to be precise via a feedback path of the phase-dependent current source.
The phase-dependent current source, in which the amplitude of the alternating current is constant for the fixed phase in each case but changes with the phase, is realized in terms of circuit engineering by a control circuit which holds a voltage constant across a constant reference element in order thus to hold constant the amplitude of the alternating current through this reference element.
If the sensor head is exchanged for another, this is generally bound up, however, with changing the measuring capacitor, since the geometrical dimensions of the sensor heads differ from one another as a rule. The result is a different characteristic between the alternating measuring voltage and the geometrical variable, so that the said known measuring device has little flexibility as to the exchange of the sensor head.
Moreover, it is frequently desired that the alternating measuring voltage or its amplitude is as linear as possible with respect to the geometrical variable to be measured, that is to say, in the present case with respect to the distance. It is possible to guarantee in this way that when the sensor head is used for distance measurement in a closed control loop of a sensor system, the dynamic response of the control loop is independent of a distance. Otherwise, in the case of excessively large distances, the distance control could be performed too slowly, and in the case of excessively small rated distances the sensor system could begin to oscillate.
A capacitive sensor head, by means of which the conditions in an ideal plate capacitor can be simulated, is used in the measuring device mentioned at the beginning for the purpose of guaranteeing the linearity between the alternating measuring voltage and the distance.
If, however, use is made of a sensor head having an electrode shape that no longer ensures the linearity between the alternating measuring voltage and the distance, then, as described above, problems occur in regulating the distance with the aid of the sensor system containing the measuring device.